The gene for a tyrosine protein kinase, p56, which is over-expressed in two murine leukemia virus-induced thymoma cell lines, will be isolated by cDNA cloning. The deduced amino acid sequence of the product of this gene will be compared to that of viral and cellular tyrosine protein kinases. So as to learn why this gene is over-expressed in these two virally-induced cell lines, the structure of the chromosome flanking these active loci will be compared to that flanking loci whose expression is more tightly regulated. The possibility that the gene has undergone rearrangement will be sought by hybridization with cloned cDNA. The possibilities of proviral integration, chromosomal rearrangement, and intra-cisternal A-type particle DNA insertion will be considered. To assess the potential of this gene to function as an oncogene, it will be inserted into a murine leukemia virus vector and the ability of the recombinant virus to transform cultured mouse cells and to induce tumors in mice will be examined. The identity of the cells which normally express this gene at a low level will be determined by hybridization of the cloned gene to RNA from a variety of murine hematopoetic cells. Finally, the possibility that altered expression of the human homologue of this murine gene plays a role in human disease will be addressed by quantification of the expression of this gene in a variety of normal and malignant human cells. These studies should reveal (1) the identity of this interesting gene, (2) the reason for its over-expression in two tumor cell lines, (3) its oncogenic potential, and (4) whether it is a gene which is also involved in the induction of malignancies in humans.